Conventional hinge units are known in which a recessed portion is formed in an outer periphery of a rotation shaft that rotates together with a first member or a second member, and the recessed portion engages with a protruding member that is biased by a biasing member to retain the first member or the second member in a predetermined rotation position. For example, see Japanese Patent Application Laid-Open (JP-A) No. 2004-23469.